The conventional automatic transmission has at least a number of forward shift ranges, as well as a parking range and a reverse range selected by a manual shift. A typical transmission of this type is an automatic transmission equipped with a plurality of friction engaging devices for attaining a plurality of shift ranges by locking the elements of a gear mechanism, which is disposed between an input shaft and an output shaft, to a case, or by bringing the elements into and out of engagement with the input shaft or output shaft, or into and out of engagement with each other, wherein these friction engaging devices include a clutch (C2) serving as one friction engaging device which takes part in achieving a reverse shift, and a brake (B2) serving as another friction engaging device which participates in achieving parking and reverse. In the automatic transmissions, a one-way clutch (F1) generally is provided for fixing one element of a gear mechanism, which is for the purpose of achieving a low-speed range, to, e.g., the case in order to effect smooth shifting from a low-speed range (e.g., first speed) to a high-speed range (e.g., second speed). The other (separate) friction engaging device (B2) is provided for causing this element of the gear transmission to participate in the reverse shift and for locking the element to the case. Where such arrangement is adopted, the output shaft is locked in a violent manner when parking is achieved while the vehicle is moving forward (in the low speed range). As a result, owing to, e.g., the inertia of a rotary body (the element of the gear mechanism), the one-way clutch is subjected to an excessive load. Therefore, to prevent this from occurring, the said other friction engaging device (B2) is often made to participate replacing the one way clutch F1, when parking is achieved.
The following is a typical example of an automatic transmission of the type aforementioned. Specifically, the transmission includes first and second planetary gear sets. The input shaft is connected to a ring gear of the first planetary gear set through a forward clutch C1, and to sun gears of the first and second planetary gear sets through a direct and reverse clutch C2. The output shaft is connected to a carrier of the first planetary gear set, and to a ring gear of the second planetary gear set. The sun gears are braked freely by a second brake B1, and the carrier of the second planetary gear is braked freely by a one-way clutch F1 and a parking and reverse brake B2.
In an automatic transmission of this kind, the clutches and brakes are selectively engaged and disengaged by hydraulic pressure to provide various shift ranges. For example, the brake B2 is engaged for parking, and both the clutch C2 and brake B2 are engaged for reverse. The torques when the clutch C2 and Brake B2 are engaged for reverse are, e.g., 1.0 and 3.5 times the input torque, respectively.
Accordingly, there is a large shock at the time of engagement if solely the clutch C2 is merely cummulatively engaged by shifting from parking to reverse.